This invention relates to leak magnetism detection sensors for use in magnetic flaw detection systems, and particularly, it relates to a leak magnetism detection sensor for the online magnetic flaw detection system that is suitable for use in leakage flux sensing for detecting surface flaws and inclusions of a steel strip by sensing the leakage flux arising from the internal and surface of the ferromagnetic object under test.
The flaw detection technique using leakage flux is a method for detecting defect by generating a magnetic field along the traveling direction of the target object and then sensing the leakage flux arising from internal and surface defects of the object. As sensors for detecting leakage flux, there are semiconductor-type magnetic sensors such as magnetic diodes, magneto-resistance devices and Hall devices, and the coil-type ones such as planar coils and induction coil sensors that hold coils wound on ferrite.
Among them, the magnetic diode has the advantages of being high in sensitivity and small in size. On the other hand, it has the disadvantages of having poor temperature characteristics, large inherit noise and low mechanical strength.
The coil-type sensors have simple structures and good temperature characteristics, while they have a disadvantage of being low in sensitivity.
Meanwhile, the Hall device, which was a low-sensitivity semiconductor magnetic sensor, has come to have improved sensitivity and temperature characteristics, thus being widely adopted as a leak magnetism detection sensor for use in the flaw detection using leakage flux.
Tin plates, which are employed in food cans, are strongly worked during the production of two-piece cans (DI cans). Thus non-metallic inclusions inside the material (hereafter, simply referred to as inclusions) cause cracks during working. The target for sensing is a volume of 0.5xc3x9710xe2x88x923 mm3, assuming an elliptic region of, approximately, 1.0 mm long, 0.1 mm wide and 0.01 mm thick. Thus there are such problems that as many as around 1000 semiconductor magnetic sensors are needed to perform an online flaw detection over the full width of the target and that a huge number of signal processing circuits for as many as 1000 channels must be prepared.
An invention similar to this invention is disclosed in Japanese Utility Model Laid-Open Publication No. Hei. 7-38956. In that disclosure, a ferromagnetic material is directly attached to the opposite side of the magnetism sensing face of a coil sensor where a coil is formed on the magnetism sensing surface, in order to enhance the sensor sensitivity by gathering leakage flux in the ferromagnetic material and rendering most of the gathered magnetic flux intersect the magnetism sensing face of the coil sensor. However, it does not refer to the expansion of detection coverage of the sensor.
Japanese Patent Laid-Open Publication No. Hei. 4-296648 has also disclosed a ferromagnetic jig installed near a magnetic sensor. This jig is a magnetic shield prepared for reducing magnetic flux around the magnetic sensor so that the high density of magnetic flux in the surrounding space bypasses the magnetic sensor and thereby the magnetic sensor may not saturate. This jig is expected to improve sensor sensitivity to some extent but does not aim to expand the detection coverage.
This invention has been made to solve those conventional problems and aims to reduce the number of sensors and signal processing circuits by expanding the detection coverage of each leak magnetism detection sensor.
In order to solve the problems mentioned above, according to this invention, a leak magnetism detection sensor, which is used in a magnetic flaw detection system that generates a magnetic field along a traveling direction of a target strip and detects online leakage flux arising from internal and surface defects of the target strip with a flaw detection head equipped with a number of magnetism sensing devices arrayed across the width of the target strip to provide signals indicating the existence of defects, has a soft-magnetic material that is installed on an opposite side of the magnetism sensing face of a magnetism sensing devices and is larger than the magnetism sensing face.
Further, each of the magnetism sensing devices is located apart from the soft-magnetic material and has another soft-magnetic material kept contact with the opposite side of the magnetism sensing face of the magnetism sensing device.
Still further, the magnetism sensing device has another soft-magnetic material kept contact with its magnetism sensing face.
Yet further, the magnetism sensing device is a Hall device.
In addition, the present invention provides a method of detecting online flaws in strips, using a leak magnetism detection sensor comprising a number of magnetism sensing devices arrayed across a width of a target strip for detecting leakage flux arising from internal and surface defects thereof and a soft-magnetic material that is installed on an opposite side of a magnetism sensing face of the magnetism sensing devices and is larger than the magnetism sensing face.
FIG. 1 shows the leak magnetism detection sensor of the present invention (for example, a semiconductor sensor). In this leak magnetism detection sensor, a soft-magnetic material 14 that has a permeability much higher than that of air and is larger than a magnetism sensing face 12A of a magnetism sensing device (for example, a Hall device) of a semiconductor magnetic sensor 12 is installed on the opposite side of this magnetism sensing face 12A, namely, on the-other side of the sensor facing the target strip, apart from this magnetism sensing face 12A at a predetermined distance, namely apart from the magnetism sensing device. Then the soft-magnetic material 14 attracts leakage flux F arising from an inclusion 10A in a steel strip 10. As a result, the coverage of each sensor becomes larger and its sensitivity is enhanced because more of the magnetic flux F is concentrated to intersect the magnetism sensing face 12A in the direction perpendicular to both magnetism sensing face 12A and strip 10.
As shown in FIG. 2, the semiconductor magnetic sensor 12 shown in FIG. 1 may have a magnetism sensing device 12C such as a Hall device on the surface of a soft-magnetic material 12B such as ferrite. Namely, the soft-magnetic material 12B is kept contact with the opposite side of the magnetism sensing face of the magnetism sensing device 12C. Seen from the target strip 10, the semiconductor magnetic sensor 12 holding the magnetism sensing device 12C on the side facing the strip 10 and the soft-magnetic material 14 are installed in this order. Then since further more of the leakage flux F is concentrated to intersect the magnetism sensing face 12A at right angles as shown in FIG. 3, the detection coverage of each sensor is expanded and its sensitivity is enhanced in this further preferable example. Reference numeral 16 in FIG. 2 denotes a supporting plate for sensor mounting.
Higher sensitivity is provided by the following mechanism. When the magnetism sensing device 12C such as a Hall device is mounted on the soft-magnetic material 12B such as ferrite as shown in FIG. 2, more of the leakage flux F gathered by the aforementioned soft-magnetic material 14 is further gathered in the magnetism sensing face 12A by the soft-magnetic material 12B used in the semiconductor magnetic sensor 12. As a result, more of the leak flux F comes to intersect the magnetism sensing face 12A in the direction normal thereto.
As shown in FIG. 4, the semiconductor sensor may have the magnetism sensing device 12C such as a Hall device on the surface of the soft-magnetic material 12B such as ferrite and may sandwich this magnetism sensing device 12C such as a Hall device between soft-magnetic materials 12B and 12D such as ferrite by installing a soft-magnetic material 12D like ferrite adhered to the magnetism sensing face 12A of the magnetism sensing device like a Hall device. As a result, the magnetism sensing face is protected and the sensitivity is further raised since the leakage flux F intersects the magnetism sensing face 12A in the direction normal thereto.
It is preferable to employ a Hall device as the magnetism sensing device 12C. Since the Hall device has a low noise level, it can easily detect such small inclusions that are referred to in this invention. In addition, since a small Hall device can be made very thin, it enables to downsize the leak magnetism detection sensor of the configuration in accordance with this invention particularly in the direction perpendicular to the surface of the strip. As a result, the Hall device makes it easier to shorten the distance between the magnetism sensing face and the target strip during measurement and to raise sensing accuracy, compared with other magnetism sensing devices.
In contrast, the leakage flux presents a distribution shown in FIG. 5 in the conventional configuration having a sensor element alone. Leakage flux F crosses the magnetism sensing face 12A of the sensor 12 at shallow angles. Then since the portion of magnetic flux normal to the magnetism sensing face 12A is small, the detection coverage per sensor is small.